The present invention relates to a photographic mirror reflex camera comprising a rangefinder, where a photoelectric detection system is mounted behind a grating acting as a spatial frequency filter in a focusing plane of the image taking objective within the rangefinder, the output signals from the detection system providing information about the degree of focus of the objective on the object being aimed at.
The state of the art of such rangefinding systems in mirror reflex cameras is described for instance in U.S. Pat. No. 3,953,728, the disclosure of which is incorporated herein. The essential characteristic of this kind of system is that the beam required for focusing is reflected through the pentaprism by specular means mounted therein and out its exit toward a grating located in front (as seen in the direction of the object), and onto a photoelectric detection system.
U.S. Pat. No. 3,953,728 discloses two kinds of specular means, namely fully reflecting mirrors mounted laterally at the beam exit surface, and a partially reflecting mirror extending at the same site across the entire beam exit surface.
However, both kinds of specular means have limitations. Thus the laterally mounted fully reflecting mirrors cover only two diametrically opposite partial regions of the exit pupil, and the full surface partially transmitting mirror sensibly decreases viewer brightness and contrast. Further, an observer's eye is reflected by this specular surface and the energy output for the photoelectric detection system is decreased.